the normally oxygenated heart. 9 Myocardial stunning is caused by myocyte dysfunction. Generation of free radicals, resulting in damage to cellular proteins and phospholipids, and disruption of intracellular calcium homeostasis, are the two, non-mutually exclusive mechanismsesponsible forhisost- injury dysfunction. 33 Subsequently, disturbances in energy production or energy use, or both, by the contractile elements can be demonstrated. However, not only does the normal functioning of the immature myocyte differ from that of the adult, but the mechanism of the cellular response to injury is also different. The first part of this review examines the major physiological differences between myocytes from immature and adult hearts, and how those differences may affect the response to reversible injury. These age-related differences also dictate that the methods we use to prevent stunning, the subject of the second half of this review, may need to vary from those used in the adult.